When certain sorbents are brought into contact with combustion gases containing sulfur compounds, the sulfur compounds react and can be removed. The efficiency of sulfur removal is often lower than that predicted from equilibrium constants. E.g., when limestone is introduced into a combustor, the efficiency of sulfur removal is typically 30-40% because the limestone particles develop a shell of calcium sulfate and the interior of the particles remains unreacted. Improving the efficiency of sulfur removal is desirable because this reduces the quantity of solids that must be disposed of or processed, thereby lowering costs, reducing harmful environmental effects, and improving energy efficiency. Most sulfur removal is based on limestone or calcium compounds because of their low cost and availability.
There are a variety of ways to reduce sulfur emissions produced during combustion. One way is to use solvents to remove the sulfur before combustion. This has not been economical. Another approach, removing sulfur compounds after combustion by scrubbing them from the flue gas, has high capital and high operating costs because of the large volume of combustion gases that must be handled.
Some sulfur-removal/combustion processes require the treatment or the collection of slurries or liquids. Not only is there an energy cost, there is a very large capital cost associated with such processes. For example, the use of a scrubber can add 70 to 100 million dollars to the cost of an installation. The addition of sulfur-absorbing compounds to the combustion process is especially attractive when the reaction product formed is dry. Sulfur-sorbent compounds can be removed in the same step and using the same kind of equipment that is already used to remove ash.
What is needed is a way to improve the efficiency and lower the cost of sulfur removal. Various methods have been attempted in recent years.
U.S. Pat. No. 4,922,840, Woodroffe et al., discloses a method of combustion in which the sorbent is injected into the products of combustion at temperatures of 1600.degree.-2400.degree. K. (2800.degree.-4300.degree. F.). The reaction products are captured at the optimum time. A drawback is that only specialized combustors operate at temperatures much above 1900.degree. K., (3000.degree. F.) the temperature range required to highly activate a calcium-based sorbent.
U.S. Pat. No. 4,503,785, Scocca, discloses a method for reduction of sulfur content of exit gases of combustion whereby concentrated solutions of calcium nitrate or other alkaline earth metal nitrates are injected into the fuel system prior to ignition or into the hottest part of the flame. These nitrates are ingnited with the fuel. Reactants, concentrations, and conditions are such that sulfur compounds are removed as calcium sulfates. One drawback is that, because nitrates are being used, nitrogen compounds would be produced and released, causing air pollution.
U.S. Pat. No. 4,387,653, Voss, discloses fine limestone-based sorbent agglomerated for the removal of sulfur compounds in hot gases. These agglomerates are used in fluidized bed combustion applications and are more reactive than natural limestone granules of the same size. Drawbacks of agglomerates are the expenses associated with the additional materials, including disposal costs and equipment costs.
U.S. Pat. No. 4,312,280, Shearer et al., discloses a method of increasing the sulfation capacity of alkaline earth sorbents by hydrating partially reacted particles in a fluidized bed to crack the sulfate coating, converting the alkaline earth oxide to the hydroxide and increasing surface area available for sulfur sorption.
U.S. Pat. No. 3,933,127, Arps, discloses a means for removing sulfur compounds and silicates from combustion products by passing them through a molten salt bath and removing the by-products as precipitates. As with similar molten salt baths, sulfur removal is limited by gaseous diffusion of sulfur compounds.
U.S. Pat. No. 4,185,080, Rechmeier, discloses a method of reducing emissions of sulfur compounds by combusting sulfur containing fuel in the presence of calcium carbonate or calcium magnesium carbonate to form calcium oxide or calcium magnesium oxide and calcium sulfate or calcium magnesium sulfate. The calcium or calcium magnesium reaction products are then slaked with water to form hydroxide and recycled to the combustion zone or to the combustion gases. This method is useful for limiting the generation of solid waste, but does not improve the efficiency of sulfur sorption.
As disclosed in U.S. Pat. No. 4,922,840, the efficiency of sulfur capture has an optimum temperature as well as an optimum time. Each design of the apparatus and each temperature will have associated with it a sorbent residence time that will affect efficiency.
Sulfur compounds can be reacted with calcium or calcium compounds under oxidizing conditions, reducing conditions, or partially oxidizing conditions. If the calcium is introduced in a vapor state, the first reaction is the formation of CaS. If the calcium is present as the oxide CaO and sufficient oxygen is present, CaSO.sub.4 would quickly be formed. Depending on temperature, time, humidity, and oxygen concentrations, decomposition can occur wherein sulfuroxides are released from the sorbent/sulfur compound.
Other sorbents also combine with sulfur. For example, potassium carbonate has been used successfully in magnetohydrodymanic combustors with potassium to sulfur molar ratios between 1.2 and 1 with more than 95% sulfur removal. ("SO.sub.2 Tests to Meet State of Tennessee Requirements, EPA Performance Tests for Particulate and SO.sub.2 Conducted at the DOE Coal Fired Flow Facility," D. G. Rasnake et al., presented at 28th SEAM, June 1990, Chicago, Ill. and "Emission Control by Magnetohydrodynamics," R. Attig and J. Chapman, ChemTech, page, 694, November 1988. Another advantage of potassium is that corrosion is diminished by an order of magnitude by the presence of the potassium-sulfur salts. ("MHD Bottoming Cycle Operations and Test Results at the Coal Fired Flow Facility," N. R. Johanson and J. W. Muehlhauser, presented at 2nd International Workshop on Fossil Fuel Fired MHD Retrofit of Existing Power Plant, Bologna, Italy Mar. 21-23, 1990. See also "500 Hour Superheater/ITAH Tube Corrosion in the CFFF," M. White and M. Le, 28th Symposium on Engineering Aspects of Magnetohydrodynamics, VII.5-1.